1. Field of the Invention
This invention relates to photo metering devices for single lens reflex cameras wherein the reflected light from the curtain of the focal plane shutter, or the film surface is metered to derive a light value based on which exposure control is made.
2. Description of the Prior Art
It has already been known to provide a photo metering optical system comprising a photo-cell and a condenser lens system arranged on the bottom wall of the mirror box in the camera so that the reflected light bundle from the shutter curtain, or the film surface is focused on the photo-cell by the condenser lens system. This photo metering method was generally adapted to put a relatively heavy emphasis on the central portion of the picture frame of film by making the condenser lens system to have a focal length somewhat shortened with an increase in the relative aperture. And, in the condenser lens system there was need of using a plurality of convex lenses in order to correct, among other various aberrations, particularly spherical aberration. A method of good correction of the spherical aberration by using an aspheric surface in one of the optical members was also adopted.
However, the use of a plurality of convex lenses leads to an increase in the bulk and size of the condenser lens system and to make it more difficult to perform their adjustment. The use of the aspheric surface also gives rise to a drawback that the production cost of the lens is increased.